In recent years, research and development of light-emitting elements using electroluminescence (EL) have been actively carried out. In a basic structure of such a light-emitting element, a layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By application of a voltage between the electrodes of this element, light emission from the light-emitting substance can be obtained.
Since the above light-emitting element is a self-luminous type, a light-emitting device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, and low power consumption. Furthermore, such a light-emitting element also has advantages in that the element can be formed to be thin and lightweight, and that response time is high.
In the case where the above light-emitting element is used for a light-emitting device, there are the following two methods: a method of providing subpixels in a pixel with EL layers having functions of emitting light of different colors (hereinafter referred to as a separate coloring method) and a method of providing subpixels in a pixel with, for example, a common EL layer having a function of emitting white light and color filters having functions of transmitting light of different colors (hereinafter referred to as a color filter method).
One of the advantages of the color filter method is that the EL layer can be shared by all of the subpixels. Therefore, compared with the separate coloring method, loss of a material of the EL layer is small and the number of steps needed for formation of the EL layer can be reduced; thus, light-emitting devices can be manufactured at low cost with high productivity. Furthermore, although it is necessary, in the separate coloring method, to provide a space between the subpixels to prevent mixture of the materials of the EL layers in the subpixels, the color filter method does not need such a space and therefore enables a high-definition light-emitting device having higher pixel density.
The light-emitting element can emit light of a variety of colors depending on the kind of light-emitting substance included in the EL layer. In the view of application of the light-emitting element to lighting devices, a light-emitting element that emits white light or light of color close to white and has high efficiency is demanded. In the view of application of the light-emitting element to a light-emitting device utilizing the color filter method, a high efficiency light-emitting element emitting light with high color purity is demanded. In addition, for the light-emitting element used for the lighting device and the light-emitting device, low power consumption is demanded.
Increasing the extraction efficiency of light is important for higher emission efficiency of the light-emitting element. In order to increase the extraction efficiency of light from the light-emitting element, a method has been proposed, in which a micro optical resonator (microcavity) structure utilizing a resonant effect of light between a pair of electrodes is used to increase the intensity of light having a specific wavelength (e.g., see Patent Document 1).
As a light-emitting element that emits white light, an element including a charge-generation layer between a plurality of EL layers (a tandem element) has been proposed.
In order to improve element characteristics of such light-emitting elements, improvement of an element structure, development of a material, and the like have been actively carried out.